Clearly, components must be formed in order that they provide the necessary structural or operational performance in the environment dictated by the machine or structure within which those components are utilised. In gas turbine engines certain components will operate at relatively high temperatures and sometimes in excess of the melting point of the metal from which those components are formed. This is achieved through appropriate cooling of the component but also by applying relatively high performance coatings to those components in order to achieve resistance to surface pitting at the elevated temperatures.
Turbine blades must operate at temperatures in excess of the melting point of the material from which the blade is formed. Typically, a protective coating is provided particularly at the tips of those blades where the blade is thin and subject to aerodynamic friction. One coating that is applied comprises a nickel alloy in which one of the trace elements is hafnium. Previously, this alloy was coated upon the turbine blade component by initially creating a master alloy block in a crucible. This leads to inconsistent and unpredictable losses of trace elements due to reaction with crucible walls. The nickel is heated in the crucible until it is molten and then the hafnium is added with appropriate stirring, etc in order to achieve or approach the desired distribution for the hafnium within the master alloy block. It will be appreciated that once melted and the hafnium mixed into the nickel the block is allowed to cool under controlled conditions, but despite best efforts it is not possible to obtain a good distribution of hafnium throughout the master alloy and there is generally a directional aspect to the cooling with cooling being more quickly achieved towards the crucible walls in comparison with the centre of the molten mass in the crucible. These problems are a result of trace element concentration loss of due to reaction crucible walls. These reactions are not usually predictable. In any event, hafnium reacts with the ceramic walls of the crucible, which creates problems including slag within the alloy block. Hafnium also scavenges oxygen from the chamber walls and oxidises and this again can create occlusions and slag within the master alloy block. As indicated, this approach is not ideal in that the hafnium is poorly distributed through the master alloy and that alloy may include undesirable elements. Additionally, formation of the master alloy block is a relatively massive procedure whereby a large block is formed from which only a small amount may be required at any particular time.
The increased desire to achieve higher engine efficiencies leads to turbine engines operating at higher temperatures and so a greater necessity to provide reliable and more convenient application of protective coatings such as those described above with respect to Nickel-Hafnium alloys upon turbine blade components.